The present invention relates to an air flow rate meter. More particularly, it concerns a thermal air flow rate meter for use in controlling an engine of an automobile.
In order to protect the natural environment and save resources, it is most important to decrease the harmful exhaust gas emissions of automobile engines, and to increase their fuel efficiency. For this purpose, it is essential to control the engine fuel supply and ignition precisely. To accomplish this objective, an electronic fuel injection system tends to be used most frequently. For such systems, it is particularly important that accurate information concerning the air flow rate to the engine be available. Thermal air flow rate meters are often provided to satisfy this need. The demand placed on the automobile's electric power system by such electronic systems, however, is significant, and can adversely affect its operation. Thus, it is important to decrease the overall electric power demand of such systems wherever possible. In, particular, one of the problems associated with thermal air flow rate meters is to find a way to reduce the very high voltage needed to operate them.
One important measure to lower the operating voltage of a thermal air flow rate meter is to lower the power used for heating a heat generating resistor when the air flow rate is null. To accomplish this, Japanese patent Laid-Open No. 134919 of 1988 discloses an indirect heating arrangement having a heating resistor which is used to heat a thermally variable resistor. Although such indirect heating systems can lower the operating power consumption, their response to changes of air flow is slowed by the time required for the flow of heat from the heating resistor to the heat generating resistor, especially because the output signal is taken from the heating resistor. Also, the contact resistance of the connectors to external equipment, or the input unit, causes some error in air flow rate detection in the input unit, because the output signal is a voltage. Such problems indirectly mean that the operating power is too high.
In view of the foregoing, it is an object of the present invention to provide an air flow rate meter and air flow rate detection method capable of detecting an air flow at high accuracy, even at lower operating voltage.